This invention relates generally to methods and circuitry for driving matrix displays and more particularly to ferroelectric liquid crystal displays.
The idea to use ferroelectric liquid crystals in display devices was proposed by N. Clark and S. Lagerwall, U.S. Pat. No. 4,367,924. The materials used in these devices are smectic C or H liquid crystals. These are materials wherein the molecules lie in planes and the molecules are tilted within the planes and rotate in a conical locus about the planar normal. These materials exhibit bistability, that is, can be switched between two stable states by reversing the polarity of an externally applied electric field, to make a bistable electro-optical device. Chiral smectic C materials (SmC*) further possess a permanent dipole moment that lies in the smectic planes and is normal to the long molecular axis. This dipole moment couples strongly with applied electric fields and allows the rotation angle of the molecules about planar normal to be controlled by applying an electric field parallel to the smectic planes.
A ferroelectric liquid crystal cell is formed by confining a thin layer of the material between two layers of glass. The ferroelectric liquid crystal molecules at the surface of the glass are constrained to be flat against the surface. The director field can thus adopt two uniform states. Electrodes are applied to the sheets of glass to complete the liquid crystal cell. The field applied by the electrodes will be parallel to the smectic planes if the planes are oriented normal to the surfaces of the glass. Clark et al. describe a method for attaining this orientation. The resultant structure provides a liquid crystal display device comprising a matrix of pixels that are switchable between at least two display states. By use of polarizers, the states can be made optically distinguishable.
The ability to make operable ferroelectric liquid crystal matrix display devices work has been demonstrated. A number of problems arise, however, in trying to design a practical, commercially-applicable ferroelectric liquid crystal display. Among the most important considerations is how to drive the display.
Most liquid crystals that have been employed in conventional practice for display devices are twisted nematic-type liquid crystals. Nematic liquid crystals have a positive dielectric anisotropy. When no electric field is present, such crystals form a twisted structure in the direction of the thickness of the liquid crystal layer. A group of scanning electrodes and a group of signal electrodes are conventionally arranged so as to intersect with each other and form a matrix. A time sharing driving method is employed for driving these displays. Address signals are sequentially applied to a group of common electrodes serving as scanning electrodes. Information signals are selectively applied to the signal electrodes in parallel synchrony with the address signals. Twisted nematic liquid crystals can be effectively driven by applying static DC signals to the electrodes. These signal levels are conventionally provided by a fixed resistive-capacitive voltage divider. A typical driver circuit for twisted nematic-type liquid crystal displays is the MSM5260 GSK Model 80 bit LCD dot matrix driver and associated circuitry, manufactured by OKI Electric Industry Co., Ltd., of Sunnyvale, Calif.
For a number of reasons, conventional driver circuitry of the type used for twisted nematic liquid crystal displays is not satisfactory for ferroelectric liquid crystal displays. Such a driver typically includes a serial-to-parallel N-bit shift register into which display data (for the column or X driver) and line select data (for the row or Y driver) are input. The shift register outputs the data in parallel to a latch, which passes the data to an N-wide 4:1 analog multiplexer array. Each multiplexer has two select inputs, one of which (FR) is common to all, the other (DATA) connected to receive data from the latch. The multiplexers each have four voltage level inputs in common. The static DC levels are applied to these inputs. These circuits are powered from a supply voltage V.sub.DD (typically +5 VDC) and V.sub.SSH (variable according to temperature, LCD type, and matrix size). In the MSM 5260, V.sub.DD also serves as one of the static DC reference voltage inputs (V1). The FR input is conventionally inverted in a 50% duty cycle, typically once per frame but, in some implementations, as often as once per line, to select between pairs of the static voltage inputs.
In ferroelectric displays, it is necessary to have bistable latching, that further has some threshold. In an actual matrix device there will always be some background voltages present at every pixel. It is important that these voltages not lower the contrast or change the state of a pixel but, on the other hand, that only a slightly greater voltage suffices to change the state of the pixel.
Another concern has to do with the nature of the electrical waveforms to be applied to the ferroelectric display device. In a ferroelectric device, a "+" polarity pulse switches a pixel to one state and a "-" pulse to the other. There is a threshold associated with this process that is, roughly speaking, related to the area under the pulse. Sub-threshold pulses (those that do not cause switching), however, can disturb the pixel. That is, while the pulse is applied, the director configuration distorts. In a matrix device, strobe waveforms will be applied to transparent conductive rows on one cell surface, and data waveforms to columns on the other surface. Waveforms applied to the rows and columns of the device produce a difference waveform from a particular row and column at each pixel. This difference waveform needs to be +V or -V during each time frame to set the selected pixels in a desired state. The rest of the time, the difference waveform should do nothing to the pixels. The data signals should affect the strobed row, setting the pixels in that row ON or OFF, while leaving the pixels in non-strobed rows unchanged. The rows are strobed sequentially so that a pixel sees the resultant of a strobe waveform and the data waveform for 1/N of the time (N=total number of matrix lines) and sees the data voltage alone for the rest of the time. It turns out that a continuously-applied DC voltage, even if it is quite small, will affect the pixels' state. Thus, another requirement for ferroelectric liquid crystal display devices is that the background voltage be "AC-like."
Lagerwall et al., in the Proceedings of the 1985 International Display Research Conference, IEEE, pages 213-221 (1985), have proposed a five-phase drive waveform to meet these needs (see FIG. 5). It requires that each row be addressed five times in a frame cycle: once to switch pixels in a row "up"; once to cancel the DC component of the first switching pulse; once to switch pixels in the row "down"; once to cancel the DC component of the third switching pulse; and once more to further increase the AC nature of the resultant waveform. Looking at the resultant waveforms, it can be seen that an "up" signal really goes "down-up" and a "down" signal really goes "up-down." Also, the data voltage waveform has "+" pulses followed by "-" pulses and vice versa to minimize the effect of the "background" voltage each pixel sees by making it more "AC" like. Lagerwall et al. have not, however, disclosed any practical, commercially-applicable driver for implementing the proposed multiplexing scheme in a ferroelectric liquid crystal matrix display.
There are also two objectionable features to the Lagerwall et al. waveform. One problem is that the black area of the screen may tend to flicker if the refresh rate is slow. This is because black pixels will momentarily flash white during each refresh cycle. The other problem is that the refresh cycle will tend to take a lot of time because each row must be addressed with a five-phase sequence, where each phase interval is roughly equal to the switching speed of a pixel. Thus, if a pixel can be switched in 100 microsecs, 500 microsecs per line is required for every frame update of matrix device.
Other driving methods have been proposed for ferroelectric liquid crystal displays but these do not adequately address the concerns outlined above. U.S. Pat. No. 4,548,476 to Kaneko proposes a time-sharing driving method that is designed to reduce the number of drivers required in a print-bar (1.times.N) array by making the array geometrically appear as, for example, a 3.times.N/3 matrix. An electrode matrix is formed by two groups of electrodes, oppositely spaced from one another and arranged so as to intersect one another to form matrix intersection points. One group serves as the row electrodes; the other as the signal electrodes. Voltage is applied to each row electrode in a time-sharing manner such that the voltage applied to a selected intersection point is in a direction opposite the voltage applied to adjacent intersection points. Kaneko discloses a two-level, two-phase drive scheme. This patent does not address the nature of the drivers themselves, or the problems of complex multiplexing, as set out above, in a large (i.e., M.times.N where both M and N are typically greater than 2.sup.6) matrix display.
U.S. Pat. No. 4,508,429 to Nagae et al addresses the drive issue directly and describes a method for controlling a switch for producing a two-level waveform (+V.sub.p and -V.sub.p) and DC-offset variations thereof to switch and periodically refresh the state of the liquid crystal, in a scheme that provides an average voltage level of zero. It does not adequately deal with the above-described concerns, however, because it does not involve bistable liquid crystals. Also, the form of drive circuit disclosed would, apparently have to be duplicated for each row and column of the matrix. This is not a practical solution for a large matrix display. The circuit also does not provide for a multitude of levels and durations.
Accordingly, a need remains for a suitable driver for complex waveform multiplexing of a bistable ferroelectric liquid crystal matrix display.